Cold cathode gas discharge device



Feb. 5, 1963 B. G. FIRTH com cATEoDE GAS DISCHARGE DEVICE Filed Sept.l0. 1959 ATTORNEYS United States Patent Ohlice 3,t"l6,9l3 @GLD CtllltthGAS Dlllrai'l Elli/ECE Bernard G. Firth, Newark, NJ., assigner to'lungadol Electric linea, a corporation ol Delaware Filed Sept. lll,w59, Ser. No. 839,299 7 iairns. tl. Zilli-wield) The present inventionrelates to cold cathode gas discharge tubes and comprises a novel threeelectrode tube that is of miniature size, delivers high peak currents ofshort duration, has short deionization periods and high hold-oilvoltages. Although the new tube is particularly adapted for use inignition circuits for righ compression combustion engines it may beadvantageously employed in any circuit where its special characteristicsare important, as for example in switching circuits, exciters andpulsera. The invention comprises novel structural and dimensionalfeatures which insure desired tube characteristics.

The cathode of the new tube comprises a metal cup within which is acathode pill and covering which is a centrally apertured metal cap ofrefractory material. rl`he trigger grid comprises a cylindricalconstruction of graphite critically spaced from the cathode cup. Theanode comprises a shielded rod of graphite connected to an anode leadextending through a top bulb seal. A generally cylindrical insulator ofceramic material surrounds the cathode cup and grid.

The cathode pill is formed by compression of a mixture of cesiurnchloride and aluminum filings. An important feature of the invention isthe range of particle size and the relative proportions of theconstituents of the mixture from which the pill is formed and whichinsures, in the linished tube, that reaction will be maintained betweenthe components of the pill. Other important features of the inventionare the spacing between the upper suriace of the pill and the cover ofthe cathode cup, the material of the cup cover, the dimensions of thecentral aperture therein and the spacing of the trigger grid from thecover. These features contribute to stability of operation of the tubeby insuring that the tube will not go into continuous glow, that thedischarge will not be choked, that the deionization time will lie thatdesired and that desired hold-oil voltages may be obtained.

The electrode spacing of the new tube, when designed as a switch tubefor discharge of the storage capacitor of au ignition circuit, is suchas to provide high forward hold-oil voltage and short deionization time.When designed as a pass tube for the reverse current through the primaryof the ignition coil following the spark discharge, the spacing of theelectrodes is such as to insure high reverse hold-ofi voltages. Thecircuit of Short et al. Patent 2,846,992 dated August l2, 1958, istypical of ignition circuits in which the cold cathode gas tubes of theinvention can be advantageously employed. That circuit includes a switchtube and a pass tube connected as above indicated.

ln both the switch and pass tube constructions, vents through theceramic cathode insulator are provided for release of gas and ofcombustion products from the cathode pill. ln the switch tubeconstruction, vents are also provided through the wall of the cathodecup, which vents communicate with the vents in the insulator.

Other features of the invention, including multiple leadin conductorsfor the cathodes to handle the large peak currents passed by the tubes,will become apparent as the description proceeds.

For a better understanding of the invention and of speciiic embodimentsthereof, reference may be had to the accompanying drawing of which:

dhld patented Feb. 5, i953 FlG. l is a longitudinal sectional View,partly in elevation, of a switch tube embodying the invention; and

FIG. 2 is a similar longitudinal section, partly in elevation, of a passtube embodying the invention.

The cold cathode gas tube embodying the invention and shown in FlG. 1comprises an envelope 2 of glass or the like having a button stern 4through which extends a plurality of cathode leads 6 and a grid lead 3.An anode lead lo is sealed through a bulb top seal. Within the envelopeis a cathode 1.2 comprising a cup 11'- of nickel within which isdisposed a cathode pill 16. A centrally apertured cover l of molybdenumis provided for the cathode cup. The cathode l2 is mounted within acylindrical recess in a cylindrical insulator 2t) of ceramic materialsuch as lava. Mounted in a second recess in the insulator 2li above thecathode and axially aligned therewith, is a control or trigger grid. 22of graphite. A metal ribbon 2li of nickel encircles` the grid 22 and iswelded to the end of the lead S, the lead 8 extending through a passagein the insulator 2i? and through an aligned hole in a mica spacer 26.The part of the grid lead d between the insulator Ztl and button stem 4of the envelope is protected by a ceramic tube 2d, the upper end olwhich is introduced into an aperture in the insulator and the lower endof which is fused or cenented to the glass of the stein.

The upper end of the envelope is provided with a reentrant tubularportion 3o within which is a thin nickel sleeve 32 having an upperilared end embedded in the wall of the reentrant portion. The lower endof the anode lead litt extends into the sleeve 32 and is welded on itsend to the graphite anode A thin short sleeve 3-5 of nickel surroundsthe junction of the lead lo with the anode 3d. Surrounding the reentrantportion of the tube is a second insulating cylinder 33 of ceramicmaterial, such as lava, which extends below the end ot the anode and isprovided with section di) of greater diameter having a cylindricalrecess for reception of the upper end of the grid 22. The grid is formedwith a central bore l2 terminating at its upper end in a section @la oflesser diameter.

radial prssages are formed in the insulator 2li hese passages terminatewith their centers precisely he lev l the top of the cathode cup ld.Formed wall of the cathode cup are six holes of smaller h aligned withthe passages 415. The holes and passages permit simultaneous ventingboth above and below the surface of the cover 13 of gas and products ofcombustion. This insures release of pres sure due to thermal expansionand rapid deionization.

The envelope is filled with helium gas at a pressure of approximately l2mm. and then sealed oit at a tubulation indicated at The spacing betweenelectrodes in the above described structure is of critical importance.rthe longitudinal distance between the top cap i8 of the cathode cup andthe lower surface of the grid 22 is determined by the minimum of thePaschen curve for the pressure of the particular gas in the tube. Inother words the spacing should be such that the sparking; potential is aminimum for the particular pressure of the specitic gas in the envelopein accordance with Paschens law (see page 164 of Gase-ous Conductors byi ames Dillon Cobine, published in i941 by McGraw-Hill). Such spacing,for the particular tube illustrated in FIG. l and for helium at apressure of l2 mm. is .030 inch. ln the same tube the spacing of theanode to the cathode is .410 inch. The spacing between the underside ofthe cap of the cathode cup and the cathode pellet is also of importance.ln the particular embodiment of the invention illustrated in FlG. l suchspacing is .03() inch, the pellet being .G inch deep and of a diameterol .257 inch. lt is in the space between the pellet and the cap of thecup that the eerdere a holes i8 are drilled -to provide escape for theproducts of combustion during conduction by the tube.

As heretofore indicated the construction of the cathode pellet i6 isimportant to insure proper operation and long life of the device. Thepellet is composed of particles of cesium chloride and aluminum filingsthat have been compressed under vhigh pressure.

When a reaction occurs between the cesium chloride andthe aluminumfilings a channel is termed in the pellet and products of combustion aregiven ott. if the reaction is to `continue the size of the channel mustbe sufficient to permit the discharge to reach additional particles ofthe pellet. The size of the larger particles determines the channelsize. Also to maintain reaction there must be small particles becausethese particles react more rapidly. Moreover, there must be a thoroughmixing or" the cesium chloride and aluminum. lf the aluminum filings arenot properly interspersed with the cesium chloride particles aconductive barrier could form around cesium chloride agglomeraties thusisolating them from possible use. In accordance with the invention, whenthe pill is made according to the following specications an interlockingelectrically conductive mesh of aluminum is established throughout thepellet of such dimensions as to allow cesium chloride' to be reacted anderoded into channels large enough to expose Vfresh surfaces tonewdischarges.

The particlesize should be within the range of -40 to +200 mesh, that isall the particles should pass through i401 mesh and none through a 200mesh. The particle size should be so distributed in the speciiied rangethat about one half of the particles lie in the range -40 to +1.00 andthe other one half lie in the range of 100 to +200 range. Such adistribution of particle size, when the components of the mixture arethoroughly intermixed gives a minimum channel size of .020 inch which issuliicient to insure penetration oity thepellet. To insure evendistribution ofy the particles they `are shaken together in a metal tubeand then quickly poured into a mold for application of pressure. Glasscontainers should be avoided. as electrostatic charges build up whichtend to separatel the constituents of the mixture. The pressure employedfor. forming vthe pellet should be about 23,000 to 29,000 lbs. persquare inch, or, for a pellet of the specified dimension, of the orderof 1200 to 15001bs. The cesium chloride particles should be slightlymoistened by short time exposure to air in a cabinet with some moisturepresent at room temperature. Tube life is enhanced when slight moistureis present but care should be taken to avoid accumulation of too muchmoisture by the hydroscopic cesium chloride particles.

The channel size necessary for penetration of the pellet is applicablealso to the spacing of the cathode cap. ln order to insure that thereaction will spread to the periphery of the pellet such spacing shouldbe at least .020 inch. One additional feature of the cathode pellet isof iniportance. The ratio of the aluminum to the cesium chloride shouldbe controlled and preferably these components should be in the ratio ofabout 2.7 parts by weight of cesium chloride to about l part by weightof aluminum.

The cap 18 of the cathode cup has a central aperture 52 therein and thisaperture, in the construction of FIG. l, is .054 inch in diameter. Sucha dimension is large enough-to insure against choking of the dischargeand at the same time small enough to insure high hold-oli voltage and toavoid the tendency of the tube to fall into continuous glow. The cap isspot welded about its periphery to the cup and, as heretofore indicated,is preferably of molybdenum or other refractory material. Molybdenum ispreferred because when wellcleaned it has a slight gettering action andbecause it oxidizes slightly during operation of the tube, thusminimizing reliection of heat into the cathode cup. A refractorymaterial is desired to avoid sputtering, such as occurs when the coveror cap is of a metal such as nickel. The edges of the aperture 52 shouldbe burr free.

A tube such as that of FIG. 1 must carry very high peak currents of theorder of 170 amperes. For this reason the plurality of cathode leads 6are provided. In the view of the drawing only three of the leads arevisible but in the complete tube there are six leads leading fromseparate pins to the base of the cathode cup. When fewer leads of largerdiameter are used difficulty is experienced with the seal of the heavierwires through the button base and also heat is generated as a result or"skin effect when the current builds up rapidly. By the use of relativelythin leads connected to provide parallel paths these difiicultiesinherent in a tube which has to withstand the sudden surge of currentoccurring in the tube of the invention are avoided.

A tube constructed as disclosed in FIG. 1 with the dimensions heretoforespecified has the following characteristics:

Max. anode voltage volts 2000 Min. anode voltage at 400 volts peaktrigger voltage volts- 1000 Max. average anode current (approx.) ma 40Max. peak anode current (approx.) amps 170 Min. peak anode current(approx.) amps..- 50 Max. anode pulse width (approx.) iisec-- 2 Tubevoltage drop (approx.) volts..- Max. repetition rate p.p.s 500 Min.trigger peak voltage-.. volts 300 Min. trigger peak current (approx.)-ma 20 Max. trigger` peak current (approx.) ainps-- 50 Recommendedtrigger source impedance ohms max 500 Max. average grid current l nia 2The tube shown in FIG. 2, the pass tube, is of construction similar tothat of the switch tube of FIG. l except for the differences now to bepointed out. The diierences are due to the different desired functioningofthe tubes, For a switch tube it is desired that there be rapiddreionization time, and a high hold-oil voltage in both the forward andreverse directions whereas in the pass tube a. high stand-off voltage inthe reverse direction only is required. To decrease the arc drop andincrease tube life the spacing between the anode and cathode is lessinthe pass tube than in the -switch tube. Speciiically, in the tube orFIG. 2, this spacing is .360 inch compared to .410 inch in the switchtube. The grid to cathode spacing is the same in both tubes as is thesize of the aperture in the cap of the cathode cup and the spacing ofthe pellet below the cap. The other major dilerence in the constructionsof the two tubes is that in the case of the pass tube of FIG. 2 the veutpassages 46 in the insulator 20 are spaced slightly above the cap of thecathode cup and no holes are provided in the wall of the' cathode cup.As in the case of the switch tube the cathode is connected by aplurality of leads 6, preferably 6 in number, through the buttonrstem ofthe envelope to separate pin terminals.

The invention has now been described in connection with two embodimentsthereof differing in relatively small respects. It will be apparent fromthe foregoing description that the invention comprises a high voltagecold cathode gas tube of construction which insures stable operation andlong useful life. The construction of the cathode pellet from thespecified range of particle size and distribution of particle sizewithin such range and of the preferred proportions of cesium chlorideand aluminum filings is of particular importance in insuring the desiredoperation of the new tube.

T he following is claimed:

l. A cold cathode gas discharge device comprising a sealed envelopecontaining inert gas at reduced pressure and having mounted therein acathode, a grid and an anode, said cathodecomprising a metal cupcontaining a compressed mixture of cesium chlorideand aluminum particlesof a variety of sizes within the range of -40 to +200 mesh, and acentrally apertured refractory Imetal cover on the cup and spaced fromthe mixture therein through a distance greater than the diameter of achannel formed in the mixture upon reaction between the particles, saidgrid being of graphite and of generally cylindrical form with a .centralpassage therethrough axially aligned with the central aperture in thecover, the lower surface of said grid being spaced from said cathodethrough a distance substantially that at which the sparking potential isa minimum for the gas and the pressure within the envelope, said anodebeing of graphite and positioned above said grid in axial alignmenttherewith, and connections `for said grid, cathode and anode sealedthrough said envelope.

2. The cold cathode gas discharge device according to claim l whereinsaid envelope has a button base carrying a plurality of pin terminalsand wherein said cathode cup is connected to a plurality of saidterminals by said connections.

3. The cold cathode to claim 1 where said molybdenum.

4. The cold cathode gas discharge device according to claim 1 includinga ceramic shield having a central opengas -discharge device accordingcover of the cathode cup is of 6 ing therethrough, said cathode and gridlacing mounted in said opening, said shield having radial vent passagestherethrough terminating at their inner ends in the neighborhood of thecover of the cathode cup.

5. The cold cathode gas discharge device according to claim 4 whereinthe walls of the cathode cup above the level of the compressed mixtureof particles has apertures therein communicating with said ventpassages.

6. The cold cathode gas discharge device according to claim 1 whereinabout half of the particles of said compressed mixture are of sizeswithin the range of 4G to and the remainder are of sizes within therange of +10() to +260.

7. The cold cathode gas discharge device according to claim 1 whereinthe compressed mixture comprises substantially 2.7 parts by weight ofcesium chloride to l part lby weight of aluminum iilings.

References Cited in the iile of this patent UNITED STATES PATENTS

1. A COLD CATHODE GAS DISCHARGE DEVICE COMPRISING A SEALED ENVELOPECONTAINING INERT GAS AT REDUCED PRESSURE AND HAVING MOUNTED THEREIN ACATHODE, A GRID AND AN ANODE, SAID CATHODE COMPRISING A METAL CUPCONTAINING A COMPRESSED MIXTURE OF CESIUM CHLORIDE AND ALUMINUMPARTICLES OF A VARIETY OF SIZES WITHIN THE RANGE OF -40 TO +200 MESH,AND A CENTRALLY APERTURED REFRACTORY METAL COVER ON THE CUP AND SPACEDFROM THE MIXTURE THEREIN THROUGH A DISTANCE GREATER THAN THE DIAMETER OFA CHANNEL FORMED IN THE MIXTURE UPON REACTION BETWEEN THE PARTICLES,SAID GRID BEING OF GRAPHITE AND OF GENERALLY CYLINDRICAL FORM WITH ACENTRAL PASSAGE THERETHROUGH AXIALLY ALIGNED WITH THE CENTRAL APERTUREIN THE COVER, THE LOWER SURFACE OF SAID GRID BEING SPACED FROM SAIDCATHODE THROUGH A DISTANCE SUBSTANTIALLY THAT AT WHICH THE SPARKINGPOTENTIAL